On Wed, 28 Sept 2022 at 17:47, Cédric Le Goater <c...@kaod.org> wrote:
>
> As the Cortex A7 MPCore Technical reference says :
>
> "When FPU option is selected without NEON, the FPU is VFPv4-D16 and
> uses 16 double-precision registers. When the FPU is implemented with
> NEON, the FPU is VFPv4-D32 and uses 32 double-precision registers.
> This register bank is shared with NEON."
>
> Modify the mvfr0 register value of the cortex A7 to advertise only 16
> registers when NEON is not available, and not 32 registers.
>
> Signed-off-by: Cédric Le Goater <c...@kaod.org>
> ---
> target/arm/cpu.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/target/arm/cpu.c b/target/arm/cpu.c
> index 7ec3281da9aa..01dc74c32add 100644
> --- a/target/arm/cpu.c
> +++ b/target/arm/cpu.c
> @@ -1684,6 +1684,10 @@ static void arm_cpu_realizefn(DeviceState *dev, Error
> **errp)
> cpu->isar.id_isar6 = u;
>
> if (!arm_feature(env, ARM_FEATURE_M)) {
> + u = cpu->isar.mvfr0;
> + u = FIELD_DP32(u, MVFR0, SIMDREG, 1); /* 16 registers */
> + cpu->isar.mvfr0 = u;
> +
Architecturally, Neon implies that you must have 32 dp registers,
but not having Neon does not imply that you must only have 16.
In particular, the Cortex-A15 always implements VFPv4-D32
whether Neon is enabled or not.
If you want to be able to turn off D32 and restrict to 16
registers, I think you need to add a separate property to
control that.
thanks
-- PMM
